Two tips for unit testing Android libraries

December 24, 2019  3 minute read  

I’m busy writing a networking library for one of my Android apps. The question today is “how do I properly test the library?” There are a few problem areas, and I’ll cover two of them today.

  1. How do I properly mock classes that aren’t really Android specific, like android.location.Location and
  2. How do I properly mock the Android context?

Android Studio already integrates the excellent JUnit packages for unit testing. I don’t want to have to write a visual app just to test my library.

Tip #1: Use Unmock to mock Android library classes

There are quite a few classes in the Android standard library that are not Android specific. When you consider it, the android.location.Location is storage for the result of an Android operation (finding location via the built in GPS), but it doesn’t touch the Android OS. My library takes a Location object as a parameter to one of the methods. I want to test the method, so I have to mock the object.

To do this, use Unmock - a library that does this for a whole host of standard classes. First, edit the project-level build.gradle file, and add the appropriate line:

buildscript {
    ext.kotlin_version = '1.3.61'
    repositories {
    dependencies {
        classpath ''
        classpath "org.jetbrains.kotlin:kotlin-gradle-plugin:$kotlin_version"
        classpath 'de.mobilej.unmock:UnMockPlugin:0.7.3'

Then edit the build.gradle for the module containing the tests. Add the plugin at the top:

apply plugin: ''
apply plugin: 'kotlin-android'
apply plugin: 'kotlin-android-extensions'
apply plugin: 'de.mobilej.unmock'

unMock {
    keep "android.location.Location"
    keep ""
    keepStartingWith "org."
    keepStartingWith "libcore."
    keepAndRename "java.nio.charset.Charsets" to "xjava.nio.charset.Charsets"

# Rest of the build.gradle file

I want to mock as well since I parse a Uri. Using this class instead of the more normal class allows me to pass it directly to an Intent to start the web browser. The other classes that are included were required by the two main classes. Unmock can mock a whole host of classes. There is an exhaustive list on their web site.

Now you can write your unit tests to use these two classes that wouldn’t normally be available.

Tip #2: Use Mockito for mocking the Android Context

Quite a few libraries - mine included - use the Android Context to read strings from a values file (like strings.xml). I use this to localize the error messages produced by the library, for instance. To do this, I call context.getString(id). Unfortunately, Unmock does not mock the context. I’m guessing most libraries don’t need the complete support of that the context provides - just a small part of it.

Enter Mockito - a library that mocks objects for you. Using this library in Kotlin is not straight forward because the major method is a reserved word in Kotlin. However, without it, you tend to jump through hoops. For example, you might create an interface with just getString() in it, then create another object that just wraps the context. This is ugly and means you can’t pass the context directly to the library - something that most Android developers expect to do.

Start by adding the Mockito library as a test dependency in your module-level build.gradle file.

dependencies {
    # Rest of the gradle dependencies

    testImplementation 'org.mockito:mockito-core:2.23.0'

You can create the mock anywhere during the test creation. I have a string referenced by the id R.string.dslib_api_key, which I mock like this:

@Test(expected = InvalidApiKey::class)
fun test_api_key() {
    va api_key = "1234"
    var context = mock(
    val actual = DSLibrary(context)
    // This should throw

You have to surround the when method with back-ticks because when is a reserved word in Kotlin. Thanks Mockito! I hope they return to whenever as a method name or provide both as alternatives, as this tripped me up for a while.

There is one more problem I am going to be solving - how to mock the network connectivity stack. However, that is a bigger blog post, so I’ll leave that for next time.

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